API RP 581 - 3rd Ed.2016 - Add.2-2020 - Risk-Based Inspection Methodology

3.A-10 API RECOMMENDED PRACTICE 581
3.A.3.6.6
3.A.3.6.6.1
Development of Toxic Consequence Areas for H 2S
General
H2S is processed as a vapor or when processed under high pressures, quickly flashes upon release due to
its low boiling point. In either case, the release of H2S to the atmosphere results in the quick formation of a
toxic vapor cloud. For H2S releases, the Level 1 consequence analysis uses the following guidelines to
determine the release rate or mass of mixtures containing H2S.
a) If the released material contains H2S as a component in a mixture, the mass fraction of H2S is obtained,
and If the initial state of the material is a vapor, the mass fraction of H2S is used to obtain the vapor
discharge rate (or mass) of only H2S; this rate (or mass) is used to determine the impact area.
b) If the initial state of the material is a liquid, the mass fraction of H2S is used to obtain the vapor flash rate
(or mass) of only the H2S; this rate (or mass) is used to determine the impact.
c) If the initial phase of a material being released is 1 wt% H2S in gas oil, the material has the potential for
both toxic and flammable outcomes from the vapor and flammable outcomes from the liquid. Therefore,
the following procedure is followed, using C17-C25 as the representative material.
1) Calculate the liquid discharge rate for C17-C25 as described in Part 3, Section 4.3.
2) When estimating flammable consequences, calculate the potential flammable consequence areas
as in Part 3, Section 4.3 and take the worst case between:
i) the flammable effects of C17-C25 using 100 % of the release rate,
ii)
the flammable effects of H2S based on 1 % of the release rate.
3) Calculate the toxic effects of H2S, using 1 % of the release rate.
For instantaneous releases, use the above procedure, substituting inventory mass for release rate.
The release durations used to model the consequences of the H2S release were identical to those assumed
for HF acid as discussed in Section 3.A.3.6.5.2.
3.A.3.6.6.2
Continuous Releases
The results of the dispersion analyses showed that the clouds modeled in accordance with the approximated
shapes of Section 3.A.3.6.3 could be correlated as functions of release rate for continuous releases in
accordance with Equation (3.A.7).
The values of the constants
Table 4.11.
c and d are functions of the release duration and provided for H2S in Part 3,
3.A.3.6.6.3
Instantaneous Releases
The results of the dispersion analyses showed that the clouds modeled in accordance with the approximated
shapes of Section 3.A.3.6.4 could be correlated as functions of release mass for instantaneous releases in
accordance with Equation (3.A.8).
For instantaneous releases, the values of the constants
c and d are provided for H2S in Part 3, Table 4.11.
3.A.3.6.7
3.A.3.6.7.1
Development of Toxic Consequence Areas for Ammonia
General
To estimate the consequence area for ammonia, the dispersion analyses was performed using a saturated
liquid at ambient temperature [24 °C (5 °F)], with liquid being released from a low pressure storage tank. The
tank head was set at 3.05 m (10 ft).